Magnesium is a cellular element essential for bioenergy transformation and macromolecular structures. It is required for bone formation, nerve conduction, muscle contraction and sodium and potassium homeostasis and has been implicated in heart disease. However, direct experimental data on magnesium variation between individual cells and magnesium distribution within a cell have not been available. This work applied the new method of computer controlled, electron beam x-ray microanalysis to quantitate magnesium variation between individual cells and study subcellular magnesium distribution. Preliminary data indicate that the magnesium content in individual lymphocites can be determined and that subcellular magnesium distribution can be imaged. The relative amount of magnesium in individual cells has been shown to vary. Experiments are underway to study synchronized cell populations to determine magnesium variation as a function of mitosis.